Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates generally to space cooling systems and in particular to
apparatus for controlling the defrost operation in a space cooling system.
Background Art
Space cooling systems, including both refrigeration and comfort cooling
systems,
typically include one or more evaporators in heat exchange relationship with a
space to
be cooled, a condenser external to the space, a compressor for circulating a
heat transfer
medium, such as a vapor compression refrigerant, between the evaporator and
the
condenser, and an expansion valve located between the condenser outlet and the
inlet to
each evaporator. Each expansion valve may be positionable at various
intermediate
positions between a fully open position and a fully closed position to
regulate the flow
rate of the heat transfer medium through the evaporator. An indoor fan is
usually
provided to direct a flow of cooling air across the evaporator and an outdoor
fan is
usually provided for cooling the condenser.
Modern-day space cooling systems may also include a microcomputer
programmed to control operation of the system based on inputs from
°i~~s
temperature and pressure sensors. Each expansion valve may be controlled in
resp~i
to the measured temperature differential across the corresponding evaporator.
This
temperature differential is commonly referred to as the evaporator superheat.
Various
techniques for controlling the expansion valve in response to evaporator
superheat are set
forth in U.S. Patents 4,067,203; 4,523,435; 4,617,804; 4,620,424; 4,674,292;
4,787,213; and
5,551,248.
Space cooling systems also typically include some type of mechanism which is
operable to prevent frost build-up on the evaporators) and a device for
controlling
operation of the defrost mechanism. Defrosting may be accomplished in a number
of
different ways, including using an electrically resistive heating element to
heat each
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evaporator, introducing hot gas into the evaporator, or operating the indoor
fan to melt
the frost accumulated on the evaporator. The defrost operation may be
initiated based
on a pre-programmed time between successive defrost operations, or,
alternatively, in
response to selected indicators, such as evaporator temperature, ambient air
temperature,
optical detection of frost build-up, compressor run time, or evaporator
cooling fan
performance. The defrost operation may be terminated in response to a
predetermined
elapsed time since the onset thereof, or, alternatively, in response to an
indication that the
evaporator temperature has reached a predetermined target temperature. Prior
art
examples of defrost controls therefor are shown in U.S. Patents 4,338,790;
4,406,133;
4,573,326; 4,882,908; 5,315,835; and 5,415,005; and in European Patent
Application EP
0 501 387/B1.
One of the problems associated with prior art defrost controls, particularly
those
in which the defrost operation is initiated at regular time intervals, is that
the defrost
operation may be initiated at times when there is really not a need to defrost
the
evaporator(s). Further, even if there is a need for defrosting, there may be
certain times
of day (i.e., periods of peak cooling requirements) during which it is not
desirable to
interrupt normal system operation in order to defrost the evaporator(s).
There is therefore a need for improved apparatus for controlling the defrost
operation in a space cooling system.
Disclosure of Invention
In accordance with the present invention, apparatus is provided for
controlling the
defrost operation in a space cooling system of the type having a first heat
exchanger in
heat exchange relationship with the space to be cooled, a second heat
exchanger external
to the space, a circulating device for circulating heat transfer fluid between
the first and
second heat exchangers, a regulating device located between the first and
second heat
exchangers for regulating heat transfer fluid flow rate through the first heat
exchanger,
and a defroster operatively associated with the first heat exchanger.
In accordance with a feature of the invention, the control apparatus activates
the
defroster in response to both of the following conditions having been
satisfied: (i) a
demand for defrost is indicated by a change in a selected one or more system
operating
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parameters indicating degradation in performance of the first heat exchanger
due to frost
build-up thereon; and (ii) the demand for defrost occurs at a predetermined
allowed
defrost time. In accordance with one embodiment of the invention, degradation
in
performance of the first heat exchanger is indicated by reduced heat transfer
fluid flow
rate through the first heat exchanger.
In accordance with another feature of the invention, the control apparatus
includes
means responsive to the absence of a demand for defrost for determining at
each allowed
defrost time whether defrosting can be deferred until the next allowed defrost
time based
on the then current rate of degradation in performance of the first heat
exchanger. In
accordance with one embodiment of the invention, the determining means
includes
computing means for computing first and second ratios, the first ratio having
a numerator
which represents degradation in performance of the first heat exchanger
compared to a
predetermined reference performance and a denominator which represents a
predetermined allowed degradation in performance of the first heat exchanger
compared
to the reference performance, and the second ratio having a numerator which
represents
time elapsed since a last defrost operation and a denominator which represents
a time
interval from the last defrost operation to a next allowed defrost time. The
determining
means further includes comparing means for comparing the first and second
ratios. The
control apparatus is further operable to activate the defroster at an allowed
defrost time
in response to the first ratio being greater than or equal to the second
ratio. Therefore,
in accordance with this feature of the invention, the control apparatus
determines, in the
absence of a demand for defrost, whether defrosting can be deferred until the
next allowed
defrost time, based on the then current rate of degradation in performance of
the first heat
exchanger.
In accordance with yet another feature of the invention, the control apparatus
adjusts for changes in the selected one or more system operating parameters
which are not
attributable to frost build-up on the first heat exchanger.
In accordance with a preferred embodiment of the invention, the heat transfer
fluid flow regulating device is a valve and the degradation in performance of
the first
heat exchanger is determined by monitoring the position of the valve. The
control
apparatus is operable to activate the defroster in response to an indication
that the valve
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is in a more closed position than a predetermined defrost target position and
to deactivate
the defroster in response to a predetermined condition having been satisfied
(e.g.,
maximum defrost time has elapsed or the temperature of the first heat
exchanger has
reached a predetermined target temperature). As the performance of the first
heat
exchanger continues to degrade due to frost build-up thereon, the valve is
moved to a
more closed position to maintain a desired rate of heat transfer fluid flow
through the
first heat exchanger. When the valve closes below the defrost target position,
it indicates
frost build-up to the extent that there is a need to defrost the first heat
exchanger.
The defrost control apparatus of the present invention combines the advantages
of a so-called "demand defrost" control system with the advantages of a
defrost control
system which inhibits defrosting during certain time periods. By monitoring
changes in
the heat transfer fluid flow rate through the first heat exchanger, preferably
by
monitoring the position of the flow rate regulating valve, the control
apparatus initiates
the defrost operation when there is a demand therefor, but only if the demand
occurs at
a predetermined allowed defrost time. Further, the control apparatus may
initiate
defrosting at an allowed defrost time, even in the absence of a demand for
defrost, if it
determines that the rate of degradation in performance of the first heat
exchanger is such
that it is advisable to defrost the evaporator without waiting until the next
allowed
defrost time.
Brief Description of Drawinet
FIG. 1 is a schematic of a space cooling system, including a controller for
controlling the system defrost operation, according to the present invention;
FIGS. 2-10 are flow diagrams, depicting the sequence of operation of the
defrost
controller, according to the present invention.
Best Mode for Carrying O~t the TnvPntinn
In the description which follows, like parts are marked throughout the
specification and drawings with the same respective reference numbers. The
drawings are
not necessarily to scale and in some instances proportions may have been
exaggerated in
order to more clearly depict certain features of the invention.
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Referring to FIG. 1, a space cooling system 10 is depicted. System 10 includes
a
first heat exchanger (e.g., an evaporator 12) in heat exchange relationship
with an indoor
space to be cooled (e.g., a refrigerated compartment), a second heat exchanger
(e.g., a
condenser 14) external to the space, a circulating device for circulating heat
transfer fluid
(e.g., a compressor 16 for circulating a vapor compression refrigerant)
between evaporator
12 and condenser 14, and a regulating device (e.g., an expansion valve 18)
located between
an outlet side 17 of condenser 14 and an inlet side 19 of evaporator 12 for
regulating the
flow rate of the heat transfer fluid through evaporator 12. A microcomputer-
based
controller 20 is provided to control operation of system 10. An indoor fan 22
is provided
for directing ambient air in the space to be cooled across evaporator 12. An
outdoor fan
24 is provided for directing outdoor air, which acts as a cooling medium,
across condenser
14. Evaporator 12 and condenser 14 are both heat transfer coils, preferably
with multiple
passes, as illustrated in FIG. 1.
Expansion valve 18 is positionable in a fully open position to allow
refrigerant to
enter evaporator 12 substantially unimpeded, in a fully closed position to
substantially
inhibit refrigerant from entering evaporator 12, and in a plurality of
intermediate
positions between the fully open position and the fully closed position to
regulate the
flow rate of refrigerant through evaporator 12. Expansion valve 18 may be of
the type
operated by an electrically operable solenoid (not shown) or an electrically
operable step
motor (not shown). In either case, expansion valve 18 is adjustable in
selected increments
to regulate the flow rate of refrigerant through evaporator 12.
First and second temperature sensors 26, 28 are respectively positioned on
inlet
side 19 and on an outlet side 21 of evaporator 12 for sensing the temperature
differential
across evaporator 12. The temperature differential across evaporator 12
corresponds to
a level of superheat of the refrigerant as it passes through evaporator 12. A
third
temperature sensor 30 is located on a discharge side 23 of compressor i6 for
measuring
compressor discharge temperature (or outdoor ambient air temperature when
compressor
16 is not operating) and a fourth temperature sensor 32 measures the ambient
air
temperature of the space to be cooled. Temperature sensors 26, 28, 30, 32 are
preferably
thermistors.
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A fifth temperature sensor 34, also preferably a thermistor, is provided for
sensing
the temperature of evaporator 12 and an electrically resistive defrost heater
36 is provided
to heat evaporator 12 to melt frost build-up thereon when system 10 is
operated in a
defrost mode. Alternatively, hot gas may be introduced into evaporator 12 to
melt the
frost thereon during the defrost mode.
Evaporator 12, expansion valve 18, controller 20, indoor fan 22, temperature
sensors 26, 28, 32, 34, and defrost heater 36 are typically housed in an
indoor unit 38,
which is defined by the dashed lines in FIG. 1. Condenser 14, compressor 16,
outdoor
fan 24 and temperature sensor 30 are typically housed in an outdoor unit.
Controller 20 preferably includes a microcomputer of the ST62T25 type,
manufactured and sold by SGS-Thomson Microelectronics, of Phoenix, Arizona,
and a
control board having a plurality of input and output connections. Controller
20 controls
various functions and components of system 10 in response to inputs from
various
sensors, including temperature sensors 26, 28, 30, 32, 34. One such function
controlled
by controller 20 is the system defrost mode, whereby from time to time heater
36 is
activated to apply heat to evaporator 12, whereby frost on the external
surfaces of
evaporator 12 is melted. The operation of controller 20 to control the defrost
mode will
be described in greater detail hereinbelow with reference to FIGS. 2-10.
Referring to FIGS. 1, 2 and 3, controller 20 (FIG. 1) performs an Establish
Baseline Valve Position Routine 100 (FIG. 2) upon system power up, which will
now be
described in greater detail with reference to FIG. 3. Pursuant to step 101,
controller 20
determines whether a baseline position of expansion valve 18 has been set
since the last
defrost operation. If it has, Routine 100 is exited. If it has not, controller
20 determines,
pursuant to step 102, whether the space temperature, as measured by sensor 32
(FIG. 1),
is below a~predetermined setpoint temperature. If it is not, controller 20
then determines,
pursuant to step 103, whether at least two hours have elapsed since the last
defrost
operation. If at least two hours have not elapsed, controller 20 exits Routine
100. If
either the space temperature is below the setpoint temperature (step 102) or
at least two
hours have elapsed since the last defrost operation (step 103), the current
position of
expansion valve 18 (FIG. 1) is stored as the baseline position, pursuant to
step 104. The
baseline valve position is used as a reference position in determining the
existence of a
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demand for defrost condition. In order to establish a valid reference
position, the space
temperature should either be at or below the setpoint temperature or at least
two hours
should have elapsed since the end of the last defrost operation, to allow the
operation of
system 10 (FIG. 1) to reach a relatively steady state. Further, the baseline
position is
established only once between successive defrost operations (step 101).
Pursuant to step 105, the space temperature, as measured by sensor 32 (FIG.
1),
and the temperature measured by sensor 30 (FIG. 1) are stored. The temperature
measured by sensor 30 on discharge side 23 of compressor 16 (FIG. 1)
corresponds to the
temperature of the heat transfer fluid on discharge side 23 when compressor 16
is
operating and to the outdoor ambient air temperature when compressor 16 is not
operating. It is important that the temperatures measured by sensors 30, 32 at
the time
that the baseline valve position is established be stored because changes in
these
temperature parameters may affect the baseline valve position and necessitate
an
adjustment thereto, as will be described in greater detail with reference to
FIG. 9. A
defrost target valve position is then established with reference to the
baseline valve
position, pursuant to an Establish Defrost Target Valve Position Subroutine
110, which
will now be described in greater detail with reference to FIG. 4.
Referring to FIGS. 1 and 4, controller 20 (FIG. 1) performs Subroutine 110
(FIG.
3) to determine a defrost target position of expansion valve 18 (FIG. 1) with
reference to
the baseline valve position. Pursuant to step 111, controller 20 determines
whether the
space temperature setpoint (i.e., the setpoint of sensor 32) is below a
predetermined
temperature (e.g., 25°F). If it is not, controller 20 assigns, pursuant
to step 112, a
predetermined nominal percentage (e.g., 20%) below the baseline valve
position, which
corresponds to a position of expansion valve 18 that is 20% more closed than
the baseline
valve position. The position of expansion valve 18 is adjusted incrementally
in selected
steps, the step sizes being variable. For example, expansion valve 18 may be
positionable
at 255 discrete binary coded positions, with position 0 corresponding to the
fully closed
position, position 255 corresponding to the fully open position and the
positions between
0 and 255 corresponding to intermediate positions between the fully closed
position and
the fully open position. For example, if the baseline valve position is set at
100 and the
defrost target position is set 20% below the baseline position, the binary
coded position
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corresponding to the defrost target position would be position 80 {i.e., 20%
below
position 100). The defrost target valve position is calculated accordingly,
pursuant to step
113 and Subroutine 110 is exited.
If, however, controller 20 determines that the space temperature setpoint is
below
25°F (step 111), controller 20 determines, pursuant to step 114,
whether the duration of
the last defrost operation was too long (e.g., more than 35 minutes). A
relatively long
defrost time indicates that degradation in the performance of evaporator 12
(FIG. 1) at the
onset of the last defrost operation was greater than an acceptable level of
degradation,
thereby resulting in a longer than acceptable defrost time. Controller 20 uses
this
information to adjust the defrost target position to a more open position,
pursuant to step
115. Typically, the defrost target position is adjusted in increments of 2%.
For example,
if the current defrost target position is 20% below the baseline position, the
defrost target
position is adjusted so that the new defrost target position is 18% below the
baseline
position, which corresponds to a position which is 18% more closed than the
baseline
position. Assuming a baseline position of 100, the new defrost target position
would be
82.
If the duration of the last defrost operation was not too long (e.g., not more
than
35 minutes), controller 20 then determines, pursuant to step 116, whether the
duratif.
of the last defrost operation was too short (e.g., less than 15 minutes). A
relatively short
defrost time indicates that the degradation in performance of evaporator 12 at
the onset
of the last defrost cycle was less than an acceptable levEl of degradation,
thereby
in a shorter than acceptable defrost time. In that case, controller 20 adjusts
the ar... . ~;
target position to correspond to a more closed position of expansion valve 18,
pursuar
to step 117. For example, if the current defrost target position is 20% below
the baseline
position, the defrost target position is adjusted so that the new defrost
target position is
22% below the baseline position, which corresponds to a position that is 22%
more closed
than the baseline position. Assuming a baseline position of 100, the new
target position
would be 78. The defrost target position will not be adjusted by more than two
increments (4%) either above or below the nominal defrost target position.
Therefore,
if the nominal defrost target position is 20%, the defrost target position is
adjustable
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between 16% (e.g., position 84 for a baseline position of 100) and 24% (e.g.,
position 76 for
a baseline position of 100).
Referring to FIGS. 1, 2, 5 and 6, controller 20 (FIG. 1) performs a Check
Allowed
Defrost Time Periods Routine 120 (FIG. 2), which will now be described in
greater detail
with reference to FIG. 5. Pursuant to step 121, controller 20 recalls the
stored allowed
defrost time periods, for which controller 20 is user-programmable. The
allowed defrost
time periods are typically programmed as predetermined time intervals (e.g.,
20 minutes)
during which the defrost operation may be commenced. If controller 20
determines,
pursuant to step 122, that the then current time is not an allowed defrost
period, Routine
120 is exited. If the then current time does correspond to an allowed defrost
time period,
controller 20 performs a Check For Defrost Subroutine 130, which will now be
described
in greater detail with reference to FIG. 6.
Pursuant to step 131 (FIG. 6), controller 20 determines whether a minimum time
(e.g., two hours) has elapsed since the last defrost operation. If a minimum
time has not
elapsed since the last defrost operation, Subroutine 130 is exited. If a
minimum time has
elapsed since the last defrost operation (step 131), controller 20 performs a
Check
Minimum Defrost Frequency Subroutine 140, which will now be described in
greater
detail with reference to FIG. 7.
Pursuant to step 141 (FIG. 7), controller 20 recalls the cumulative cooling
run-
time since the last defrost operation. Controller 20 then determines, pursuant
to step 142,
whether the cumulative run-time since the last defrost operation has been
greater than 24
hours. If so, heater 36 (FIG. 1) is activated to defrost evaporator 12 (FIG.
1), pursuant to
a Defrost Coil Subroutine 160, which will be described in greater detail
hereinafter with
reference to FIG. 10. If not, Subroutine 140 is exited. Therefore, pursuant to
Subroutine
140, controller 20 ensures that defrosting occurs at least once every 24
hours, irrespective
of whether there is a demand therefor.
If the cumulative run-time since the last defrost operation has not been
greater
than 24 hours, controller 20 performs a Check For Initial Defrost Subroutine
150, which
will now be described in greater detail with reference to FIG. 8. If an
initial defrost
operation has already occurred (step 151), controller 20 exits Subroutine 150.
If the initial
defrost operation has not yet occurred, controller 20 determines, pursuant to
step 152,
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whether a predetermined minimum cooling run time (e.g., four hours) has
elapsed since
system power up. If it has not, Routine 150 is exited. If the minimum cooling
run time
has elapsed, controller 20 activates heater 36 (FIG. 1) to accomplish the
initial defrost
operation, pursuant to Defrost Coil Subroutine 160. Therefore, pursuant to
Subroutine
150, controller 20 ensures that the initial defrost operation occurs within a
predetermined
time (e.g., four hours) after system power up.
Referring again to FIG. 6, if Subroutine 140 and Subroutine 150 are both
exited
without performing Defrost Coil Subroutine 160, controller 20 then recalls,
pursuant to
step 132, a defrost target valve position established pursuant to Subroutine
110 (FIG. 4)
and determines, pursuant to step 133, whether the then current position of
expansion
valve 18 (FIG. 1) is lower (i.e., in a more closed position) than the defrost
target position.
If it is not, Subroutine 130 is exited. If it is, controller 20 indicates a
demand for defrost,
pursuant to step 134, and Subroutine 130 is exited. One skilled in the art
will recognize
that in order for a demand for defrost to be indicated, both of the following
conditions
must have been satisfied: (i) the current position of expansion valve 18 is
lower (i.e., in a
more closed position) than the defrost target valve position determined
pursuant to step
113 in FIG. 4; and (ii) a minimum time (e.g., two hours) has elapsed since the
last system
defrost operation.
Referring again to FIG. 5, if Subroutine 130 indicates a demand for defrost,
controller 20 determines, pursuant to step 123, that there is a need for
system defrost and
performs a Check/Adjust Defrost Target Valve Position Subroutine 180, which
will now
be described in greater detail with reference to FIG. 9.
Pursuant to step 181 (FIG. 9), controller 20 recalls the stored space
temperature
(step 105 in FIG. 3). Controller 20 then determines, pursuant to step 182
whether the
current space temperature is 5°F or more below the recalled stored
space temperature.
If it is, controller 20 next determines, pursuant to step 183, whether the
defrost target
valve position has been adjusted since the last defrost operation. If it has
not, the current
space temperature is stored in lieu of the recalled stored space temperature,
pursuant to
step 184. If the current space temperature is not 5°F or more below the
recalled stored
space temperature or if the defrost target valve position has been adjusted
since the last
defrost operation, the current space temperature is not stored.
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If the current space temperature is not stored in lieu of the recalled stored
space
temperature, controller 20 then recalls the stored outdoor or compressor
discharge
temperature (the temperature stored pursuant to step 105 in FIG. 3), pursuant
to step 185.
Controller ZO then determines, pursuant to step 186 whether the current
outdoor or
compressor discharge temperature is 10°F or more above the recalled
stored space
temperature. If it is, controller 20 next determines, pursuant to step 187,
whether the
defrost target valve position has been adjusted since the last defrost
operation. If it has
not, the current outdoor or discharge temperature is stored, pursuant to step
188, in lieu
of the recalled stored outdoor or compressor discharge temperature.
If the current outdoor or compressor discharge temperature is not 10°F
or more
above the recalled stored outdoor or compressor temperature (step 186) or if
the defrost
target valve position has been adjusted since the last defrost operation (step
187), the
current outdoor or discharge temperature is not stored and Defrost Coil
Subroutine 160
is executed. If either the current space temperature or the current outdoor or
discharge
temperature is stored, pursuant to step 184 or step 188, the current position
of expansion
valve 18 is used as the baseline position, pursuant to step 189. If the
current position of
expansion valve 18 is used as the new baseline position, a new defrost target
valve position
is calculated, pursuant to step 190, using the valve closure percentage
determined pursuant
to step 112, step 115 or step 117 in FIG. 4. Subroutine 180 is then exited.
Referring again to FIG. 5, if a demand for defrost is not indicated, pursuant
to step
123, but the current time corresponds to an allowed defrost time (step 121),
controller 20
determines whether the current rate of degradation in performance of
evaporator 12 is
such that defrosting should be accomplished now or can be deferred until the
next
allowed defrost time. Pursuant to step 124, controller 20 calculates a first
ratio, the
numerator of which is the current degradation in performance of evaporator 12
compared
to a predetermined reference performance and the denominator of which is a
maximum
allowed degradation in performance compared to the reference performance.
Controller
20 calculates a second ratio, pursuant to step 125, the numerator of which is
time elapsed
since the last defrost operation and the denominator of which is the time
between the last
defrost operation and the next allowed defrost time. The first and second
ratios are
compared, pursuant to step 126. Controller 20 determines, pursuant to step
127, whether
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the first ratio (coil degradation ratio) is less than the second ratio (time
ratio). If it is, then
controller 20 determines that defrosting can be deferred until the next
allowed time.
However, if controller 20 determines that the first ratio is greater than or
equal to the
second ratio, pursuant to step 127, it indicates that the current rate of
degradation in
performance of evaporator 12 is such that defrosting cannot be deferred until
the next
allowed time. As a result, controller 20 initiates Check/Adjust Target Valve
Position
Subroutine 180, as described hereinabove with reference to FIG. 9.
Referring to FIGS. 1 and 10, Defrost Coil Subroutine 160 will now be described
in greater detail. Pursuant to step 161, controller 20 recalls the stored
allowed defrost
time periods. If the then current time corresponds to an allowed defrost time
period (step
162), controller 20 determines, pursuant to step 163 whether the space
temperature has
dropped below 36°F. If it has not, controller 20 determines, pursuant
to step 164,
whether the coil temperature of evaporator 12 has dropped below 25°F.
If it has not,
Subroutine 160 is exited. If either the space temperature has dropped below
36°F (step
163) or the coil temperature of evaporator 12 has dropped below 25°F
(step 164), the
normal cooling mode of system 10 is terminated, pursuant to step 165 and the
defrost
mode of operation is commenced, pursuant to step 166.
Pursuant to step 167, controller 20 recalls a stored defrost termination
temperature, which corresponds to the coil temperature of evaporator 12, as
measured
by sensor 34 (FIG. 1), at which the defrost operation is to be terminated. If
the current
coil temperature of evaporator 12 is above the stored termination temperature
(step 168),
the defrost operation is terminated, pursuant to step 169. If the coil
temperature of
evaporator 12 is not above the termination temperature (step 168), controller
20 recalls
a stored defrost termination time duration (step 170), which corresponds to
the maximum
allowed time of the defrost operation. Controller 20 then determines, pursuant
to step
171, if the termination time duration, which is user-programmable, has been
set for less
than 45 minutes. If it has, controller 20 uses 45 minutes as the termination
time duration,
pursuant to step 172. If the termination time programmed by the user is not
less than 45
minutes, the program termination time is used and controller 20 determines,
pursuant to
step 173, whether the duration of the defrost operation has exceeded the
termination
time. If it has not, controller 20 branches back and continues to check for a
condition
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indicating termination of the defrost operation. If the defrost operation has
exceeded the
allowed time (step 173), the defrost operation is ended, pursuant to step 169.
One skilled in the art will recognize that the defrost operation is terminated
if the
coil temperature of evaporator 12 exceeds a predetermined temperature (i.e.,
the
termination temperature, pursuant to step 168), or if the duration of the
defrost operation
exceeds a predetermined duration (i.e., the termination time duration,
pursuant to step
173), whichever occurs first. After the defrost operation has been terminated,
pursuant
to step 169, the defrost time duration is stored, pursuant to step 174, and
Subroutine 160
is exited.
In accordance with the present invention, an improved defrost controller is
provided for a space cooling system. The defrost operation is initiated only
in response
to a demand therefor and only at a predetermined allowed time. A demand for
defrost
is indicated by a change in a selected one or more system operating parameters
indicating
degradation in evaporator performance due to frost build-up thereon. In
accordance with
a preferred embodiment of the invention, a demand for defrost is determined by
monitoring the position of the expansion valve at the evaporator inlet. As
frost builds
up on the evaporator, the expansion valve gradually closes to maintain a
desired level of
superheat across the evaporator. When the expansion valve is closed to a
position below
a predetermined defrost target position, a demand for defrost is indicated.
The present
invention also makes allowance for changes in system operating parameters,
such as space
temperature and compressor discharge temperature, which may affect the
position of the
expansion valve in a way which is not related to frost build-up on the
evaporator. The
system controller takes these changes into account and adjusts the defrost
target valve
position accordingly.
Various embodiments of the invention have now been described in detail. Since
it is obvious that changes in and additions to the above-described best mode
may be made
without departing from the nature, spirit or scope of the invention, the
invention is not
to be limited to said details, but only by the appended claims and their
equivalents.
